1. Field of the Invention
The present invention relates to an ignition apparatus for an internal combustion engine which has a plurality of transformers arranged in a casing, and which supplies a high voltage to a spark plug for each engine cylinder.
2. Description of the Related Art
FIG. 11 is a cross sectional side view of an ignition apparatus 1 for an internal combustion engine shown in Japanese Utility Model Publication No. Hei 8-5540. FIG. 12 is a cross sectional view taken along line XII—XII of FIG. 11.
The ignition apparatus 1 illustrated is used by a simultaneous ignition system for an internal combustion engine, and includes three transformers each having a secondary transformer connected at its opposite ends to two spark plugs for use with an internal combustion engine having six cylinders.
The first through third transformers 1A, 1B and 1C of the ignition apparatus 1 for an internal combustion engine are accommodated in a casing 8 made of a resin with high-voltage towers 8a being arranged at its head. Each of the first through third transformers 1A, 1B and 1C includes a coil part 100 which is provided with a closed magnetic circuit core 2 forming a hollow-rectangular-shaped closed magnetic circuit, a primary coil 4 with a conducting wire being wound around a primary bobbin 3 surrounding an excitation portion 2a of the closed magnetic circuit core 2, and a secondary coil 6 with a conducting wire being wound around a secondary bobbin 5 surrounding the primary coil 4. Each coil part 100 is fixedly secured to an inner side of the casing 8 by a casted resin portion 12 of a thermosetting property such as an epoxy resin or the like.
The respective closed magnetic circuit cores 2 of the transformers 1A, 1B and 1C have their central axes arranged in the same direction, and the excitation portion 2a, the primary coil 4 and the secondary coil 6 for each core 2 are arranged on a side opposite an opening of the casing 8. Each closed magnetic circuit core 2 has another side portion 2f opposing the corresponding excitation portion 2a located at the opening side of the casing 8. The conducting wire of each primary coil 4 is electrically connected at its one end with an unillustrated external connection terminal through a conductor 10 arranged inside the ignition apparatus and a connector terminal 10a in a connector 9a, and it is finally connected through the external connection terminal to a power supply (not shown) such as a battery of a vehicle. The conducting wire of each primary coil 4 is finally connected at its other end with a corresponding terminal of a switching module (not shown), e.g., a collector of a corresponding one of power transistors which constitute the switching module.
The conducting wire of each secondary coil 6 is connected at its opposite ends with terminals 6a, respectively, fixedly attached to a fixed part 5a of the secondary bobbin 5 by means of soldering or the like. The terminals 6a are electrically connected with corresponding secondary terminals or high-voltage terminals 8b which are assembled to the casing 8 through insert molding or press fitting. The casing 8 has high-voltage towers 8a protruded from portions thereof in which the coil parts 100 are received in the casing 8. Connected with each secondary terminal 8b is one end of a high-tension cord (not shown) which is in turn connected at its other end with a spark plug (not shown).
With the ignition apparatus 1 for an internal combustion engine as referred to above, after the plurality of transformers 1A, 1B, 1C, the conductors 10, etc., are arranged at their prescribed positions in the casing 8, a resin such as an epoxy resin is injected into the casing 8 of a bottomed cylindrical configuration under a vacuum atmosphere and then set or cured at a high temperature in a curing oven, thus achieving the fixed mounting of the built-in components within the casing 8 and the insulation thereof against high voltages.
Now, the operation of the ignition apparatus 1 for an internal combustion engine as constructed above will be explained below.
When coil drivers (not shown) such as power transistors are driven by a control signal from a control unit (not shown) of the internal combustion engine, a primary current flowing through the primary coils 4 is controlled to be supplied and interrupted in an appropriate manner by the coil drivers. That is, the coil drivers are turned off at prescribed ignition timing of the internal combustion engine thereby to cut off the primary current of the corresponding primary coils 4, whereupon counterelectromotive forces are generated in the primary coils 4 whereby high voltages are produced in the secondary coils 6 of the transformers 1A, 1B and 1C. As a result, the high voltages thus produced are impressed on the spark plugs (not shown) connected with the secondary coils 6, whereby air fuel mixtures in the unillustrated engine cylinders are dielectrically broken down, as a consequence of which electrical discharges are caused due to a secondary current flowing through the secondary coils 6, thus firing the air fuel mixtures in the engine cylinders.
The known ignition apparatus 1 for an internal combustion engine as constructed above involves the following problems. That is, as can be seen from FIG. 11, the respective transformers 1A, 1B and 1C are arranged in such a manner that the respective planes of the hollow-rectangular-shaped closed magnetic circuit cores 2 (i.e., planes normal to the surface of the sheet of FIG. 11) in the casing 8 are disposed in a parallel relation with respect to one another. As a result, the overall height of the casing 8 becomes high. In addition, the high-voltage towers 8a of the casing 8 protrude from those portions of the casing 8 in which the coil parts 100 are received, and hence the overall height of the total casing 8 including the secondary terminals 8b and the high-tension cords connected therewith becomes large. This results in a great disadvantage to the arrangement of the ignition apparatus inside the engine room of a limited space in the vehicle.
Moreover, there are additional various problems, too, as described below. That is, because the overall height of the casing 8 becomes large, the amount of material for the casted resin portion 12 filling an accordingly increased extra space in the casing 8 is increased, thus increasing the cost of manufacture accordingly.
When the high-tension cords 13 are attached to the ignition apparatus 1 in a state of its having been assembled to the internal combustion engine 14, as shown in FIG. 13, the high-voltage towers 8a are disposed in mutually closely adjacent locations with practically no room in space therearound. Thus, the efficiency in the assembling operation of the high-tension cords 13 is poor.
Further, it is necessary to secure a special space for exclusive use with a plurality of conductors electrically connecting the primary coils 4 and the secondary coils 6 with the external connection terminals, thus resulting in an increased size of the entire ignition apparatus.
Furthermore, in cases where the switching module for switching on and off the current supplied to the primary coils 4 is incorporated in the ignition apparatus 1 or the casing 8 in order to achieve reduction in the total cost of the ignition apparatus, it is necessary to secure a special space for this purpose, thus resulting in an increased size of the entire apparatus.
Besides, although the transformers 1A, 1B and 1C in the casing 8 are fixed thereto by means of the casted resin portion 12, reliability of the apparatus is impaired due to cracks generated especially in the casted resin portion 12 at the opening of the casing 8 by thermal strain caused by a difference in the coefficients of linear expansion of the respective members resulting from a variation in the heat of the primary coils 4 and a change in the temperature of the surrounding environment.